Objectives. Alcohol use disorder (AUD) and posttraumatic stress disorder (PTSD) frequently co-occur, and having one condition worsens the course of the other. Individuals with both disorders exhibit worse functioning across a number of domains than individuals with either disorder alone. Prolonged exposure therapy (PE) is among the most effective treatments for PTSD. PE has been rated as a frontline treatment by multiple guidelines and reviews including the VA/DoD Clinical Practice Guidelines for the treatment of PTSD. However, in studies of individuals with PTSD and AUD, changes in alcohol use are only slightly better than in control or standard care conditions, reductions in PTSD symptoms are modest relative to studies of PE in PTSD patients without AUD, and rates of drop out from treatment are high. Combining PE with medication to curb drinking shows promise to improve upon the effectiveness of PE for individuals with comorbid AUD and PTSD, although thus far few studies have examined combining psychotherapy and medication. Topiramate is the single medication that has shown effectiveness for both AUD and PTSD and shows promise for reducing drinking among individuals with AUD and PTSD. However, the effect of adding topiramate to PE to treat comorbid AUD/PTSD has yet to be examined. The critical next step is to test a best practice PTSD treatment, PE, together with a promising pharmacological agent, topiramate, which has been found to be effective for both AUD and PTSD. Innovation: This application seeks to shift current clinical practice paradigms. A refinement to existing interventions is proposed through integration of two evidence based treatments. Methodology. We propose to use a randomized, controlled, double blind study design to examine the effect of adding topiramate (TOP) to a best practice treatment for PTSD, PE. Participants will be 120 male and female Veterans from all services with AUD and PTSD. Our primary aims are to determine the relative efficacy of PE+topiramate, as compared to PE+placebo, in reducing problematic drinking, reducing PTSD symptoms, and improving functioning and quality of life among Veterans with comorbid AUD/PTSD at post-treatment and 3- and 6-month post-treatment follow-up. We will explore the extent to which decreases in drinking and PTSD symptoms lead to improvement in functioning. The proposed study has the potential to improve functional and psychological recovery for a highly prevalent and highly impaired population of Veterans. This study will test a novel and innovative combination of psychotherapy and medication with the goal of improving the care of Veterans. The successful completion of this project will help change the practices that drive treatment for Veterans who have both AUD and PTSD. The fundamental rationale for this study is to improve the evidence base that informs how patients with AUD and PTSD can attain sustained recovery from both of these disorders. We will also explore whether changes in PTSD symptoms in the PE+TOP condition are partially explained by reductions in alcohol cravings. Norman - 1